If I wanted to pay Mercedes prices for used BMW parts I would've bought a MBZ Christ! $1850. and $1500 firm, for 12 year old tops with sagging headliners and deep scratches...

It's supply and demand. If I wanted a top, I would pay that.
When I was in the market for a Z3 last fall, I looked at one at a used car dealer with a hard top on a dolly. At the time, I considered the hard top of no value, an encumbrance to be stored on an expensive hoist if it was to be used at all.
So to me a hard top is worthless. But then in 2003, after a long search, I paid that much for a 40-year old fairing shipped from Canada for my vintage BMW motorcycle to allow cold weather riding.